Inbred miniature swine provide a unique preclinical model for the study oftransplantation immunity and tolerance. Overthe past 18 years, we have utilized this model to study a robust form oftolerance of MHC class I- mismatched renal allografts that is routinely achieved following a short course of calcineurin inhibitors. This proposal represents a G37 renewal ofthe previous long-standing ROl, which received a Merit Award at the time ofthe last competitive renewal. During the past three and one-half years since that renewal, we have made significant progress in understanding the role of regulatory T cells (T-reg) in determining the balance between alloreactivity and down-regulation of immune reactivity to tolerated renal allografts. Among these studies, has been an evaluation ofthe possible role ofantigen presentation through direct or indirect pathways of activation in the reinforcing oftolerance by donor-type skin allografts in previously tolerant animals from which the tolerizing graft has been removed. We have obtained considerable evidence for dominance of down-regulation by the.direct pathway in this phenomenon. In the next project period, we intend to: 1) Complete our studies on the nature of cell populations responsible for adoptive transfer oftolerance that are still in progress;2) Determine whether the thymus is required for breaking and/or reinforcing tolerance after graftectomy in animals tolerant of class I mismatched renal allografts;and 3) Examine the mechanism by which B cell immunity to class I antigens is controlled in animals tolerant ofa class I-mismatched renal allograft through evaluation of the influence of tolerance on antibody fine specificity. The broader goal ofthese studies remains to develop an understanding ofthe mechanisms by which allograft tolerance is induced and maintained in this large-animal model, in order to permit development of appropriate protocols for induction oftolerance to organ allografts in the clinic. RELEVANCE (See instructions): Immune tolerance of transplanted organs would allow patients to avoid long-term immunosuppressive drugs and the complications caused by these drugs. We have developed a large animal model in which tolerance of kidney transplants can be induced reproducibly. Our goal here is to understand the mechanism of tolerance in this model in order to eventually extend the studies to human organ transplants.